The present invention relates to an improved parabolic trough concentrating solar collector whose optical design is optimized to maximize the concentration ratio and collection efficiency. The collector, which is intended for use in a solar tracking mode, has a geometry that facilitates fabrication and is insensitive to manufacturing imperfections. The use of the high concentration ratio collector of the present invention permits a working fluid to be heated to a higher temperature than is possible with a comparable collector of low concentration ratio, and its ease of fabrication and insensitivity to fabrication errors reduces manufacturing costs and increases reliability.
In recent years, the rising interest in solar energy has stimulated a substantial effort in the design of effective solar collectors. The simplest devices intercept incident solar radiation without concentration, for example flat plate collectors. With nonconcentrating devices, a working fluid can be heated up to e.g., 90.degree. C. While such a temperature is useful in certain applications, for example domestic hot water systems, it has low thermodynamic efficiency for processes depending upon a heat engine since large temperature differences are not possible. Furthermore, industrial process heat applications often require higher temperatures.
A variety of concentrating collectors have been designed and built whose objective is to increase the solar power density incident on the absorber to permit higher operating temperatures. Among the concentrating collectors suggested heretofore are compound parabolic collectors (Winston collector), Fresnel lenses, concentrating paraboloids, inflated tube concentrating collectors, and faceted fixed mirror collectors. The concentration ratios vary from 2X to over 100X. In some cases the collectors track the daily motion of the sun across the sky and in other cases the collector positions are adjusted less frequently, e.g., with a period of weeks. There is no consensus in the solar energy field on the question of which is the "best" type of collector. The performance varies with the design and so does the cost of fabrication/installation/maintenance. Recent approaches are summarized in the proceedings of 3rd Annual Solar Heating and Cooling Research and Development Branch Contractors Meeting, Sept. 24-27, 1978, U.S. Department of Energy Report CONF-780983, March, 1979.
A parabolic trough concentrating solar collector consists of an image forming optic which is a paraboloid of translation, and a receiver or absorber located at the focal line of the paraboloid. One such arrangement, comprising a tracking parabolic trough reflector developed heretofore, is described in Antrim et al "A Parabolic Solar Reflector For Accurate And Economic Producibility", Proceedings of the Society of Photo-Optical Instrumentation Engineers, Vol. 161, p. 36 (1978), and operates in the manner to be discussed hereinafter in reference to FIG. 1 of the drawings herein. The tracking system is used to point the paraboloid at the sun so that the direct rays of sunlight are parallel to its axis. A mirror surface is manufactured, by one of several methods, and conforms to a portion of the mathematical paraboloid surface. The rays from the sun strike the paraboloid mirror and are reflected toward the receiver at the focus of the paraboloid. The concentration ratio is 12X (the theoretical limit is 67X). A single collector of this type is capable of achieving a 200.degree. C. stagnation temperature and an efficiency up to 60%, and the collectors can be disposed in a modular array consisting of, for example, eighteen such collectors.
The present invention may, if desired, make use of certain auxiliary features of the system described above, such as the tracking system, the fabrication method, and the absorber design. However the present invention employs basically different collector and absorber configurations whereby the collector has maximum concentration ratio under the condition that no rays, from reflections at any point on the paraboloidal surface, are lost, and whereby the configuration of the paraboloidal reflector is such that its performance is insensitive to manufacturing imperfections.